1. Field of the Invention
The present invention relates to a thermal flow meter.
2. Description of the Prior Art
One flow meter of such type has been disclosed in Japanese Patent Examined Publication No. Sho 60-50289. Referring now to FIG. 7 roughly showing the above described flow meter, said flow meter comprises a conduit 50, through which a sample fluid passes, provided with sensor coils 51, 52 formed of a heat sensitive resistance wire wound around the peripheral surface thereof, a bridge circuit WS with said sensor coils 51, 52 and resistances 53, 54 as sides being formed, a first closed space 55 comprising said sensor coils 51, 52 being formed outside of said sensor coils 51, 52, said conduit 50 being provided with an enveloping member 56 formed of heat conductive materials mounted thereon by the thermal jointing, said enveloping member 56 being surrounded with a second closed space 57, said conduit 50 being provided with an outside enveloping member 58, whereby forming a convection-preventing means for said first closed space 55 and said second closed space 57, and a flow rate of the fluid flowing through the conduit 50 being determined on the basis of a shift of the bridge circuit WS from the balanced state. In addition, reference numeral 59 designates an output terminal.
In the flow meter constructed in the above described manner the vicinity of the sensor coils 51, 52 is thermally stabilized by means of the enveloping member 56 provided in the vicinity of the sensor coils 51, 52 and the closed spaces 55, 57 provided with the convection-preventing means, whereby no change of an outside temperature reaching the sensor coils 51, 52 and furthermore, the heat convection within the flow meter can be prevented even though the flow meter is inclined to some extent in the installation and the like thereof, so that the sensor coils 51, 52 can operate under the stabilized temperature condition to achieve the accurate determination of a flow rate.
However, the above described prior art has the following disadvantages:
In general, a flow meter of this type has been frequently installed in a feed passage of gases used in the manufacture of semiconductors but since high boiling substances contained in said gases are adhered to an inner wall of the conduit after the use for a long time according to circumstances, it is necessary to remove said high boiling point substances by baking out with flowing inert gases, such as nitrogen, through the conduit or depressurizing whenever occasion arises. This baking out operation is carried out by heating the conduit up to an appointed temperature. For example, a ribbon heater is wound around the peripheral surface of a case including said conduit therein or a heater (for example "POSISTER" (trade name) manufactured by Murata Works, Co., Ltd.), which is formed of a self temperature control type resistor having a positive resistance temperature coefficient whose resistance value is suddenly increased at a certain design temperature or more, is closely mounted on the peripheral surface of said case including said conduit therein.
In such the prior art as shown in FIG. 7, the conduit 50 is adapted to be covered by the enveloping member 56 and the outside enveloping member 58 other than said enveloping member 56, whereby the enveloping member 56 is thermally connected with the conduit 50, so that it takes a considerably long time for a temperature of the conduit 50 to reach a temperature required for the baking out operation and as a result, it is difficult to efficiently carry out the baking out operation in a short time even though the outside enveloping member 58 is heated by for example the ribbon heater wound around the peripheral surface thereof. In addition, the conduit 50 and the sensor coils 51, 52 surrounding said conduit 50 are adapted to be doubly enveloped by the enveloping member 56 and the outside enveloping member 58, so that the problems occur also in that the construction of taking out a lead wire and the like are complicated in addition to the inferior workability and the expensiveness.